Gas analyzing equipment is utilized for many purposes and in many applications, an important one of which is the continuous measurement of a parameter of a subterranean atmosphere, e.g. the continuous monitoring of the concentration of a combustible and/or toxic component of an air current, an air stream ventilating a mine gallery or shaft.
Devices for this purpose may include a power source, e.g. a battery, a measuring system responsive to the parameter to be measured and actuating unit and a display, signalling or other data processing unit, i.e. a unit which responds to the information supplied by the measuring unit.
The measurement and evaluation of environmental parameters may be used for long term information gathering or even for short term information development which can be directly evaluated or can be stored for subsequent evaluation for any desired purpose.
The principal reason for using such systems in a mine or other subterranean environment is for safety, i.e. to alert mine personnel to a potentially dangerous situation, to initiate the evacuation of the mine or to trigger some remedial operation such as increased ventilation.
As a consequence, systems of the type described have been long used in such mining applications.
It is known, for example, to measure the concentration of explosive and combustible gases, such as fire damp or methane by a catalytic combustion measuring system, to measure toxic gas concentrations (e.g. carbon monoxide concentrations) by chemisorption on a metal oxide semiconductor, to measure oxygen concentrations by a chemical current generating system whereby the oxygen is involved in an electrolyte action, and to quantitatively determine carbon dioxide in a gas stream utilizing thermal conductivity phenomena (see the brochure 6/82 describing the "VABOTECTOR-EX" instrument marketed by the firm Gesellschaft fur Geratebau mbH & Co. KG of the German Federal Republic).
Indeed, various systems may be used for measuring a single parameter, for example, at different concentration ranges. The latter is the case where, for small percentages of methane in the atmosphere, the catalytic composition method can be used whereas, for higher concentrations which may exceed the lower explosive limit and where catalytic combustion may be dangerous, a thermal conductivity method may be applied.
Continuous measurements have also been carried out heretofore with different measuring systems. One of the problems with continuous measuring systems, at least in prior art applications, has been that the zero point, balancing and/or sensitivity tends to change significantly with time. Thus one may observe a change in the pitch or inclination of the measuring curve, variations in inflection points thereof, etc. This, or course, leads to a falsification of the measuring results.
Not all measuring systems are suitable in all applications. For example, some have a greater electric power demand than others so their use as power availability falls becomes less and less desirable. Consequently, one may wish to use one measuring system where the power availability is substantial and as power availability diminishes, to switch to another measuring system. These and other reasons, such as safety, accuracy, etc. require a variety of measuring systems to be available.
In my printed German patent document--open application No. DE-OS 32 43 542 (corresponding to German Patent Application No. P 32 43 542), which is not prior art as to this application but which is incorporated herein by reference, I have described a system utilizing different measuring systems for a given parameter in which one measuring system is utilized normally and the system is switched over to a second measuring system when an interruption in service of the first measuring system results or may result.
In this case a particularly sensitive and accurate measuring system for low concentration of methane was cut out and a less precise system substituted for it to maintain the reliability of the overall arrangement in detecting the advent of dangerous conditions in a mine. This application, however, did not deal with problems encountered with the shift in the characteristic curve reference levels, zero point or balance in a continuous measurement of such a parameter.